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Document limitation of getTimezoneOffsetMinutes helper

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Florian Egger
2026-05-09 23:47:37 +02:00
parent bbc0f312d8
commit 2737bac3a9
1 changed files with 8 additions and 3 deletions
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src/lib/hafas-time.ts
+8 -3
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@@ -2,9 +2,14 @@
// HAFAS returns times in Vienna (CET/CEST). This module provides correct parsing // HAFAS returns times in Vienna (CET/CEST). This module provides correct parsing
// regardless of the server or browser timezone. // regardless of the server or browser timezone.
// Helper: get the offset (in minutes) from UTC for a given instant in a specific IANA timezone. // Helper: get the offset (in minutes) from UTC for a given instant in a specific
// Reads the hour via Intl.DateTimeFormat in both UTC and the target timezone, then // IANA timezone.
// computes the difference. Handles midnight wraparound (e.g. UTC 23:xx → TZ 01:xx). //
// CAVEAT — Hour-level precision only. This helper reads the hour via
// Intl.DateTimeFormat (hour part only), so it returns incorrect results for
// timezones with fractional-hour offsets (e.g. India +05:30, Nepal +05:45).
// It is designed exclusively for "Europe/Vienna" (CET/CEST), which only uses
// full-hour offsets (+01:00 / +02:00). Do not reuse for other timezones.
function getTimezoneOffsetMinutes(instant: Date, tz: string): number { function getTimezoneOffsetMinutes(instant: Date, tz: string): number {
const getHour = (timeZone: string) => { const getHour = (timeZone: string) => {
const parts = new Intl.DateTimeFormat("en-US", { const parts = new Intl.DateTimeFormat("en-US", {